The present invention relates to a display device provided with a semiconductor device (such as a thin-film transistor (TFT)) and a method for manufacturing the same.
As an active layer of a thin-film transistor (TFT) serving as a driver in a liquid crystal display device, a polycrystalline semiconductor film is superior to an amorphous semiconductor film because the polycrystalline semiconductor film is so high in mobility of carrier (electrons in an n channel or holes in a p channel) that both small cell size and high resolution can be attained.
An ordinary polycrystalline silicon TFT needs to be subjected to a high-temperature process at a temperature of 1,000° C. or higher. On the other hand, in a low-temperature polycrystalline semiconductor production technique in which only a semiconductor layer is annealed by a laser so that a substrate is not exposed to a high temperature, a TFT high in mobility can be formed in a low-temperature process in which an inexpensive substrate can be used.
In the laser annealing, an amorphous silicon film 71 as a precursor film formed on a substrate 7 of a liquid crystal display device is scanned while irradiated with a laser beam 11 absorbable to the amorphous silicon film 71 as shown in FIG. 2. As a result, the whole surface of the amorphous silicon film on the substrate is polycryallized to form a polycrystalline silicon film 72. Incidentally, another amorphous semiconductor film may be used as the precursor film.
As shown in FIG. 3, the grain size of the polycrystalline silicon changes (from regions R1 to R5) according to the energy density (fluence) of laser beam irradiation. Accordingly, laser stability is reflected in the grain size distribution of the polycrystalline silicon. The carrier mobility of the polycrystalline silicon film increases as the grain size of the polycrystalline silicon increases. It is therefore necessary to keep the grain size distribution uniform and keep the grain size large so that high-performance TFT characteristic uniform in a plane can be obtained.
The grain size will be kept large if the energy density of the region R4 in FIG. 3 can be used. When the energy density however changes to a higher one because of instability of laser beam irradiation, a region containing microcrystal having a grain size of not larger than 200 nm is used as represented by the region R5. In this case, carrier mobility is reduced to cause a device failure.
The grain size changes not only according to the energy density of laser beam irradiation but also according to variation in film thickness of the amorphous silicon before laser annealing. Accordingly, the instability of laser beam irradiation and the influence of variation in film thickness on the substrate must be suppressed so that a polycrystalline silicon film with a grain size always kept in a predetermined grain size range can be produced. It is therefore important to inspect the grain size of the polycrystalline silicon, feed the result of inspection back to the laser annealing condition and control the grain size of the polycrystalline silicon to be kept constant.
A technique concerning the method for controlling the energy density of laser beam irradiation at laser annealing has been described in JP-A-10-294289. The technique is as follows. The degree of orientation of crystal in a crystallized thin film is measured with an orientation analyzer. A judgment is made on the basis of a result of the measurement as to whether the energy density of laser beam irradiation is either too much or too little. A laser beam output is adjusted on the basis of the judgment.
A technique for evaluating the grain size of the polycrystalline silicon by measuring a light diffraction pattern and feeding the evaluated grain size back to the laser annealing process has been described in JP-A-2003-109902.